Thermode cleaning method

ABSTRACT

A method for cleaning a thermode tip including applying an energy pulse to the thermode tip. The energy pulse involves raising the temperature of the thermode tip higher than the working temperature of the thermode tip. A method for cleaning a thermode tip may include periodically performing a predetermined number of soldering cycles at a working temperature; and applying an energy pulse to the thermode tip.

RELATED APPLICATIONS

This patent is a continuation of U.S. patent application Ser. No.12/954,781 filed Nov. 26, 2010, which claims priority to U.S.Provisional Patent Application 61/264,690 filed Nov. 26, 2009, both ofwhich are hereby incorporated herein by reference.

FIELD

The present document relates generally to thermode cleaning. Moreparticularly, the present document relates to a cleaning method forcleaning thermode soldering tips.

BACKGROUND

Thermodes are devices used for local application of heat and pressure,typically in soldering applications known as ‘Hot-bar reflow soldering’.Once the thermode is brought into contact with a desired location,localized heating is produced by direct resistance heating of the tip ofthe thermode. The ‘soldering gun’ is a common example.

The main advantage of thermode soldering is the very rapid temperaturechange (up to 1000° Celsius per second) with precise control over thetemperature while the component parts are being mechanically held bythermode contact pressure. Also, since the hottest portion of thethermode is typically in direct contact with the part bond area,efficient heat transfer occurs and rapid heating of the item ispossible. The bond area is the area(s) of the part being processed wherea reflow soldered bond is desired between at least two surfaces. Sincethe tips have little thermal mass, rapid cooling and solidification ofthe completed bond is possible. Forced air cooling can additionally beused to reduce the time required for the soldering cycle.

There are several styles of thermodes in common use, which differ mainlyby the shape of the tip and material used. A thermode typically includesthe following elements:

-   -   Terminals: electrical contacts where power is applied.    -   Mount: means of mechanically supporting the thermode (possibly        the same structure as supports the terminals).    -   Shank: means of supporting the tip and conducting current to it.    -   Tip: high resistance section where the majority of heat is        developed.    -   Transition zone: means of joining the tip to the shank.    -   Working surface: the portion of the tip, which comes in contact        with the item to be heated.    -   Thermocouple: a device for determining the working temperature,        attached to the tip near the working surface

Issues may arise when the thermode, especially the thermode tip,requires cleaning due to a build-up of flux/solder residue and gradualmetallurgical contamination of the working surface (collectivelyreferred to as residue or debris). Even with the no-clean fluxformulations, which may contain less than 5% solids, there is stillresidue build-up over time as the actual burn-off amount for no-cleanfluxes is generally about 50% of the initial flux weight. Cleaning offthe residue may require hours of downtime of the manufacturing systemand reduced service life, which can cost a company many thousands ofdollars of lost revenue over a year. Current methods may require thethermode soldering tips to be abrasively scrubbed or chemically cleanedevery 150 to 175 soldering cycles.

Currently, various thermode tip cleaning methods are employed; althoughnot without their disadvantages. Another common method for thermode tipcleaning is the use of a sponge soaked in water or a chemical cleaningsolution. An air blast may also be used to clean the thermode tip. Boththe air blast method and wet sponge method generally do not eliminatethe need for mechanical scrubbing of the thermode, but may somewhatreduce the frequency of the mechanical scrubbing. Each of these methodstypically requires additional tooling at the soldering site. Thus, thereis a need for a method of cleaning a thermode that overcomes at leastsome of the deficiencies of conventional systems and methods.

SUMMARY

This application relates to a method of cleaning a thermode that isintended to take less time to complete, require minimal additionaltooling or motion and improve the useful life of the thermode. Thiscleaning method is also intended not to reduce the soldering processreliability or generate additional particulates that may contaminate thelocal environment.

In one aspect, there is provided a method for cleaning thermode tipsincluding the application of an energy pulse to the thermode tip whilethe thermode tip is not in contact with a bond, such that the thermodetip reaches a predetermined temperature above a working temperature ofthe tip. This method may result in a longer interval between normalmechanical scrubbing cycles and a substantial increase in thermode life.

In another aspect, described herein, a method for cleaning a thermodetip is provided, including, periodically: performing a predeterminednumber of soldering cycles at a working temperature; and applying anenergy pulse to the thermode tip while the thermode tip is not incontract with a bond, such that the thermode tip reaches a predeterminedtemperature above a working temperature of the tip.

In the above methods, the energy pulse may result in the thermode tipreaching the predetermined temperature for a predetermined amount oftime.

In some cases of the above methods, the temperature of the thermode tipmay be raised above the working temperature during the energy pulse. Insome cases, the thermode tip may be raised to a temperature that is atleast greater than 120% of the working temperature, to at least greaterthan 200% of the working temperature, or to at least greater than 250%of the working temperature. The temperature and the amount of time thatthe energy pulse will be applied can be determined based onmanufacturing parameters and the temperature range available given thematerial of the thermode tip.

In some cases of the above methods, the energy pulse is externallyapplied.

Other aspects and features will become apparent to those ordinarilyskilled in the art upon review of the following description of specificembodiments in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments will now be described, by way of example only, withreference to the attached Figures, wherein:

FIG. 1A illustrates the front view construction of a typical thermode.

FIG. 1B illustrates the side view of the typical thermode in FIG. 1A.

FIG. 1C illustrates the perspective view of the typical thermode in FIG.1A.

FIG. 2 illustrates a flow chart of a typical thermode reflow solderingprocess.

FIG. 3 illustrates a flow chart of a typical thermode cleaning cycle aspracticed in the current state of the art.

FIG. 4 illustrates a flow chart of a thermode cleaning method accordingto one embodiment of the present application.

FIG. 5 illustrates a thermode working surface after 175 bonds since thelast performed mechanical scrubbing, using the typical cleaning cycle aspracticed in the current state of the art.

FIG. 6 illustrates a thermode working surface contamination after 375bonds since the last performed mechanical scrubbing using the cleaningmethod provided for in the present application.

FIG. 7A illustrates reflow soldering temperature profile without usingthe present method.

FIG. 7B illustrates reflow soldering temperature profile according toone embodiment of the energy pulse cleaning system.

DETAILED DESCRIPTION

In one aspect herein, a method for cleaning a thermode tip comprisingapplying an energy pulse to the thermode tips is disclosed

A typical thermode construction is illustrated in FIG. 1. The thermodeincludes terminals (1) where the power is applied and a shank (2) forsupporting and conducting current to a tip (3). A transition zone (4)allows for the joining of the tip (3) to the shank (2). The workingsurface (5) is the portion of the tip, which comes in contact with thepart bond area. The tip (3) and, in particular, the working surface (5)generally require cleaning due to residue build-up, a by-product ofrunning soldering cycles.

FIG. 2 illustrates a typical thermode reflow soldering process. Thethermode is first lowered to the bond area (10). The thermode is thenramped up to a preheat temperature (11) to the required temperaturelevel for the bond. Once the preheat temperature is achieved thethermode holds at this temperature (12). The application of the thermodeto the bond raises the bond above the liquidus temperature (13). Thethermode is then cooled, which cools the bond below the solidustemperature (14). The cooling may be facilitated by an air blow off. Thethermode tip is then moved off the bond area (15). There will generallybe a move to the next bond location (16) or a part may be moved underthe thermode location to expose a new bond location. This thermodesoldering process is typically under 6 seconds per bond. In some casesthere may be a wait (17) prior to moving to a new bond area or a newpart being ready to be soldered and the process restarting.

FIG. 3 shows a flow chart of a typical cleaning cycle. First, thethermode solders a current bond area (20), for example, as shown in FIG.2. Once the current bond area is soldered the process must determine ifthe part itself is complete (21). If there is at least one other bondarea on the current part the process will generally move to the nextbond (22), until all bonds on the part are complete.

If all the bonds on the part are completed the bond quality may bechecked (23), for example by electrical testing or the like. If the bondquality is acceptable, the method will wait for the next part (24) thenrestart the cycle. If the bond quality is determined to be inadequate, amechanical scrubbing (25) of the thermode tip may need to be performed.It will be understood that, in some cases, the bond quality may actuallybe checked while, in other cases or alternatively, a predeterminednumber of bonds may be set as the limit for when the mechanicalscrubbing will be performed. The predetermined number may be determinedin advance by conducting testing or studies of the bonds produced.

The mechanical scrubbing (25) may take upwards of 20 seconds to performbefore the thermode may come back online and continue with the solderingprocess. After the mechanical scrubbing is performed, the quality of thethermode may be checked to determine if the thermode is worn out (26) orif it is able to continue with the next soldering cycle. The checking ofthe thermode may be a visual check, a test of heating capability, orother type of test as known in the art. If the thermode is worn out,production may be stopped for maintenance (27) during which the thermodeis replaced. Stopping the production and completing the maintenance maytake significant time, sometimes upwards of half an hour. During thistime the soldering process is halted. Throughout a year, this downtimemay add up to thousands of dollars of lost revenue.

Mechanical scrubbing (25) includes manually scrubbing the thermode withan abrasive material like a scrubbing pad or sandpaper. In this process,both the residue and a portion of the tip material are typicallyremoved. This process may later compromise the mechanical structure ofthe thermode, reduce the thermal mass, and alter the electricalresistivity of the heat generating path. All of these may wear out thethermode and compromise the effectiveness of the soldering process andreduce the thermode life. Frequent mechanical scrubbing will increasethe overall process time, taking minutes to move the scrubbing tool intoposition and perform each scrubbing cycle.

Other physical cleaning techniques may also be applied either inaddition to or instead of completing mechanical scrubbing each time thebond quality is found to inadequate. For example, wet sponge method maybe used. However, this method requires a sensor to determine the wetnessof the sponge to be used to clean the thermode tip. Further, thiscleaning method must be completed when the thermode tip is hot, whichmay become a fire hazard. As with mechanical scrubbing, the wet spongecleaning may be required every cycle and may necessitate additionaltooling including a motor for moving the sponge, a water container andpump to keep the sponge wet.

An air blast may also be used to clean the thermode tip although thismethod has the possibility of generating airborne particulate that couldcontaminate parts and the local environment. With use of either the wetsponge method or air blast method, mechanical scrubbing may still berequired if the bond quality is found to be inadequate after the wetsponge or air blast cleaning have been completed.

In order to overcome at least some of the drawbacks of conventionalmethods, the present application provides a method of applying an energypulse to the thermode tip either after each soldering operation, aftereach part, after a predetermined number of soldering operations, or thelike. As shown in FIG. 4, the soldering process of the present methodcommences similarly to a conventional method, in that a current bond issoldered (30), the process determines if the part is complete (31) andif there is at least one other bond area on the current part, theprocess will move to the next bond (32).

Once a part is complete, the bond quality may be reviewed (33) and, ifthe bond quality is adequate, the method will perform an energy pulsecleaning (34). This energy pulse cleaning (34) may preferably beperformed during idle time when the thermode is otherwise not beingused, for example, waiting for a new part to enter the process.

The energy pulse consists of an elevation of the thermode tiptemperature to a predetermined temperature higher than the workingtemperature of the thermode for a predetermined period of time. In thisway, the energy pulse may be considered to be a function of time andtemperature and the amount of energy to be input, whether through hightemperature for a short time or a lower temperature for a longer time.The temperature and time can be determined based on various factors orthrough testing as will be understood in the art. In a manufacturingenvironment, the time available is typically controlled by processparameters such as part movement, number of actions performed, and thelike. As such, the temperature needed is generally at least partlypredetermined by the amount of time available. It will, of course, beunderstood that, in some cases, it may be necessary to raise thetemperature of the thermode tip for a time longer than the cycle timeavailable, however this is generally not preferred with regard tomanufacturing efficiency.

In a conventional soldering process, the thermode tip temperature istypically lowered below the working temperature when the thermode isidle (see FIG. 7A, described in further detail below). This may be, forexample, because energy can be conserved by allowing the thermode tip tocool when idle. In contrast to this conventional approach, the methodherein raises the temperature of the thermode tip above the workingtemperature using the energy pulse. Generally, it is preferred if theincrease of the temperature is to a temperature that is at least greaterthan 120% of the working temperature. In some cases, the temperature maybe greater than 150% of the working temperature. In other cases, thetemperature may be greater than 200% of the working temperature. Instill other cases, the temperature may be greater than 250% of theworking temperature. In all cases, the increase in temperature should bekept below any temperature that may damage the thermode tip. As notedabove, the time available for the energy pulse is often determined basedon other factors but may be in a range greater than 0.1 sec inaccordance with the temperature to be used. It will be understood thatthe word “pulse” as used herein is not intended to limit the amount oftime that the energy may be applied but is used for convenience becausethe time is ideally shorter than longer in terms of manufacturingefficiency.

The energy pulse is generally performed when the thermode tip is out ofcontact with the bond area so that additional heating of the bond areadoes not occur. The temperature may be raised by normal operation of thethermode and using closed loop temperature control. Alternatively, thetemperature may be elevated by exposing the tip to an external source ofenergy or heat; although this approach may require additional tooling.

In one particular example the flux used is a tin based flux that mayhave a liquidus temperature of 240°. Normal reflow temperatures forsoldering with this material may have a working temperature of 320°. Inthis example, the energy pulse is configured to raise the temperature ofthe thermode tip to approximately twice the working temperature for aperiod of approximately 10 seconds, which, in this process, was theamount of time needed to ready the system to solder the next part.

During energy pulse cleaning (34), it is believed that any solderremaining on the thermode tip is raised to a temperature sufficient forthe surface tension of the solder to form spherical beads on the workingsurface and at least a portion of the residue is burned off the tip andvaporized. It may be that with, for example, the use of a titaniumthermode, the residue does not bond well to the titanium oxide that istypically present on the thermode tip and the energy pulse allows thetitanium oxide to reform and protect the thermode tip. Other chemicalreactions may also be occurring that protect the thermode tip fromresidue.

Small amounts of combustion by-products may be created during the energypulse process. In some cases, the remaining solder and combustionby-products may stay on the thermode tip and, during the followingsoldering cycles, it is believed that the debris is transferred backfrom the thermode tip to the surface of the solder joint as the solderis melted. The debris appears to remain on the surface and it isbelieved that the debris is not transferred into the bond.

Once the energy pulse cleaning is completed the system is ready toprocess the next part (35) and begin this cycle again. If at the end ofthe next or subsequent cycles the thermode tip cleanliness is inadequate(33), mechanical scrubbing may be performed (36). After the mechanicalscrubbing the quality of the thermode tip may be reviewed (37) and ifthe thermode needs to be replaced the production may be stopped formaintenance (38).

The cleanliness of the thermode tip may be estimated or determined in avariety of ways, which will assist with determining the number of solderoperations (cycles) between energy pulses and mechanical cleanings. Forexample, the energy transfer to the bond from the thermode tip may bemonitored, either during test runs or in production, to determine anappropriate number of soldering cycles before cleaning.

Tests have shown that the energy pulse cleaning method significantlyreduces the need for mechanical scrubbing cycles. In one example, it wasfound that mechanical scrubbing was required 4 times more often onthermodes not exposed to the energy pulse. Also, thermodes that wereexposed to the energy pulse cleaning were capable of almost 3 times asmany bonds before wearing out and needing to be replaced.

The increased cleanliness of the thermode using the method herein isfurther illustrated in FIGS. 5 and 6. FIG. 5 illustrates a thermodeworking surface after 175 bonds since the last performed mechanicalscrubbing using the typical state of the art while FIG. 6 illustratesthe working surface contamination after 375 bonds since the lastperformed mechanical scrubbing using the energy pulse cleaning method.Even with over twice as many bonds, the thermode making use of theenergy pulse cleaning method contains less residue than the workingsurface of the thermode in a typical process.

FIGS. 7A and 7B illustrate the soldering temperature profiles with andwithout using the energy pulse cleaning method. In FIG. 7A, the processbegins with an idle temperature (40) followed by a ramp up (41) then ashort holding of the temperature (42) prior to the soldering commencing(43). Once the soldering is completed the traditional process cools down(44) then waits at an idle temperature (46) prior to repeating the aboveprocess. In FIG. 7B, the temperature profile for the present energypulse cleaning method commences in a similar manner in that the thermodestarts at an idle temperature (40), heats up (41) then holds at thattemperature (42) prior to starting the soldering process (43). Thepresent method then provides for an energy pulse (45) after the thermodehas completed the bonding process (44). It is this energy pulse thataids in the cleaning of the thermode and reduces the residue thusreducing the frequency for mechanical cleaning. As noted above, theenergy pulse is not necessarily applied after each soldering operation.Further, the thermode tip may not necessarily need to be cooled prior toapplying the energy pulse.

This energy pulse cleaning method allows for the energy pulse cleaningto be accomplished without additional external tooling. As the carbondebris and excess solder may remain on the thermode tip and be removedin a later soldering cycle, no debris is expelled into the air, no wetsponge cleaning or the like is required and no additional mechanicalwear and tear is experienced by the thermode tip during the energypulse. The mechanical scrubbing of the thermode tip is required at areduced frequency in the present method, which may extend service lifeof the thermodes tips.

The energy pulse method is intended to create a continuous self-cleaningmechanism, which may keep the thermode tip generally clean and free ofdebris. It has been observed that applying this method to titaniumthermodes, the titanium thermodes stayed reasonably clean even afterover 700 soldering cycles. No further scrubbing was required to dressthe thermode tip for further soldering process during these cycles.

This method is preferably used in conjunction with titanium thermodestips; although, thermode tips made of molybdenum, Inconel®, tungsten andother metals having similar properties may also benefit from thisprocess. Presently preferred thermode metals include: low resistancegrades of titanium such as commercially pure Ti in ASTM grades 1, 2, 3or 4; alloys of Ti with moderate resistivity such as ASTM grades 12, 15,17 or 9; other alloys of titanium particularly for thermodes withrelatively small tips. The use of this method with non-titaniumthermodes has not been tested in detail.

In the preceding description, for purposes of explanation, numerousdetails are set forth in order to provide a thorough understanding ofthe embodiments. However, it will be apparent to one skilled in the artthat these specific details may not be required in order to practice theembodiments. In other instances, well-known electrical structures andcircuits may be shown in block diagram form in order not to obscure theembodiments.

The above-described embodiments are intended to be examples only.Alterations, modifications and variations can be effected to theparticular embodiments by those of skill in the art without departingfrom the scope, which is defined solely by the claims appended hereto.

1. A method for cleaning a thermode having a thermode tip, the methodcomprising: applying an energy pulse to the thermode tip such that thethermode tip reaches a predetermined temperature above the workingtemperature of the thermode tip.
 2. The method of claim 1 wherein thethermode tip is held at the predetermined temperature for apredetermined amount of time.
 3. The method of claim 1 wherein theenergy pulse is externally applied.
 4. The method of claim 1 wherein thepredetermined temperature is at least greater than 120% of the workingtemperature.
 5. The method of claim 1 wherein the predeterminedtemperature is at least greater than 200% of the working temperature. 6.The method of claim 1 wherein the predetermined temperature is at leastgreater than 250% of the working temperature.
 7. The method of claim 2wherein the predetermined amount of time the energy pulse is applied isdetermined by manufacturing parameters and a temperature range availablegiven the material of the thermode tip.
 8. The method of claim 1 whereinthe predetermined temperature is determined by manufacturing parametersand a temperature range available given the material of the thermodetip.
 9. A method for cleaning a thermode having a thermode tip, themethod comprising periodically: performing a predetermined number ofsoldering cycles at a working temperature; and applying an energy pulseto the thermode tip such that the thermode tip reaches a predeterminedtemperature above the working temperature of the thermode tip.
 10. Themethod of claim 9 wherein the thermode tip is held at the predeterminedtemperature for a predetermined amount of time.
 11. The method of claim9 wherein the predetermined temperature is at least greater than 120% ofthe working temperature.
 12. The method of claim 9 wherein thepredetermined temperature is at least greater than 200% of the workingtemperature.
 13. The method of claim 10 wherein the predetermined amountof time the energy pulse is applied is determined by manufacturingparameters and a temperature range available given the material of thethermode tip.
 14. The method of claim 9 wherein the energy pulse isexternally applied.
 15. The method of claim 9 wherein the predeterminedtemperature is determined by manufacturing parameters and a temperaturerange available given the material of the thermode tip.